The present invention relates to thermotropic cholesteric liquid crystalline polypeptides, and in particular the present invention relates to polypeptides which are copolymers consisting essentially of .gamma.-benzyl glutamate and .gamma.-alkyl glutamate and have thermotropic cholesteric liquid crystalline properties.
Generally, it has been well known that there are three phases of a substance, i.e. solid, liquid and gas as modes of existence, and further a liquid crystal which is in an intermediate state between solid (crystal) and liquid has also been known. A liquid crystal can be classified as thermotropic or lyotropic. The former is thermally induced, and the latter is formed in solution with water or other solvents. Alternatively, they can be classified as smectic, nematic, cholesteric etc. from the configurations of molecules.
Heretofore, many lower molecular compounds such as ammonium oleate, 4-cyano-4'-alkoxybiphenyl, and cholesteryl acetate have been employed as liquid crystalline compounds. Recently, the research has been concentrated on polymers having liquid crystalline properties, and aromatic polyamides and aromatic polyesters are commercially used as the fibers having higher strength. In the utilization of these liquid crystalline polymers it is important how preforms the fixation of liquid crystalline state.
Among the liquid crystalline polymers special attention has been paid to the helical structure of cholesteric liquid crystal in view of the optical functionality and the lower anisotropy of mechanical strength of molded article. In order to fix the cholesteric liquid crystalline structure, there are many processes, i.e. (1) solvent is slowly evaporated from the lyotropic state to produce a solid film, (2) thermotropic cholesteric liquid crystal is glassified in the liquid crystalline state, (3) lyotropic liquid crystal using vinyl monomer as a solvent is photopolymerized and fixed and so on.
These prior processes have the following drawbacks. The drawbacks of the process (1) are that the solvents dissolving the liquid crystalline polymer are limited and the evaporation of the solvent should be carefully effected; those of the process (2) are that the rate of crystallization of liquid crystalline polymer is fast and the glassification of the liquid crystal is difficult; and those of the process (3) are that there are few photopolymerizable vinyl monomers which can dissolve the liquid crystal as in the process (1). Further these processes have not always succeeded in controlling cholesteric pitch (or pitch simply), which is the feature of the cholesteric liquid crystal, with good reproducibility.
On the other hand, in view of moldability of a polymer, the use of thermotropic liquid crystalline polymers has been expected because of the advantage that they can be employed by a slight modification of the prior molding method of thermoplastic resins. As the thermotropic liquid crystalline polymers two kinds of polymers, i.e. in main chain type and a side chain type, have been proposed based on the chemical structure of their repeating units. The former has repeating units consisting of the rigid mesogens and flexible spacers in the main chains, while the latter has side chains containing rigid mesogens linked to flexible main chains. Apart from these, as a third type, a structure which contains rigid mesogens as main chains and flexible chains as side chains has been considered, and in fact polypeptides in which two specific alkyl groups having different chain lengths are introduced into an ester moiety of the lyotropic liquid crystalline polypeptides have been found to be thermotropic cholesteric liquid crystalline polymers (J. Watanabe et al., Macromolecules 17(5), 1004 (1984)).
These polymers have drawbacks such as a higher temperature at which they show liquid crystallinity and incomplete cholesteric structure. Thus, if these drawbacks are overcome, wider application fields will be expected.